INTRODUCTION

Blood platelets are small, anucleate cellular fragments that play an essential role in hemostasis. During normal circulation, platelets circulate in a resting state as small discs (Fig. 1.1A). However, when challenged by vascular injury, platelets are rapidly activated and aggregate with each other to form a plug on the vessel wall that prevents vascular leakage. Each day, 100 billion platelets must be produced from megakaryocytes (MKs) to maintain the normal platelet count of 2 to 3 × 108/mL. This chapter is divided into three sections that discuss the structure and organization of the resting platelet, the mechanisms by which MKs give birth to platelets, and the structural changes that drive platelet activation.

THE STRUCTURE OF THE RESTING PLATELET

Human platelets circulate in the blood as discs that lack the nucleus found in most cells. Platelets are heterogeneous in size, exhibiting dimensions of 0.5 × 3.0 μm. The exact reason why platelets are shaped as discs is unclear, although this shape may aid some aspect of their ability to flow close to the endothelium in the bloodstream. The surface of the platelet plasma membrane is smooth except for periodic invaginations that delineate the entrances to the open canalicular system (OCS), a complex network of interwinding membrane tubes that permeate the platelet's cytoplasm.